The present disclosure relates to optical systems and methods. More particularly, the present disclosure is related to systems and methods for distortion correction in analog photonic links.
Due to their high bandwidth, low cost, reduced size, and immunity to electromagnetic interference, analog optical links present a desirable alternative to traditional RF or microwave cables. Notably, their first widespread application was in the cable broadcasting industry to distribute CATV signals. The low loss of optical fibers and the large distances between source and destination made it a logical choice to replace the lossy coaxial cables and the multitude of amplifiers they required. However, to gain greater acceptance across other applications, the fundamental performance of analog optical links needs improvement while maintaining an ultra-wide operating bandwidth. For instance, this may necessitate increasing spurious-free dynamic range (SFDR) while reducing loss and noise.
Generally, external intensity-modulated direct detection (“IMDD”) links are the preferred method of transmitting an analog signal through fiber due to the inherent simplicity, where the receiver is just a photodiode. On the other hand, despite the additional receiver complexity, phase-modulated interferometrically detected (“ΦMID”) links possess several distinct advantages over IMDD links, including simplicity at the transmit end, without need for biasing electronics, and balanced detection without necessitating duplex fiber spans. In particular, these are compelling advantages for radio-over-fiber and antenna remoting applications. However, while the phase modulation itself is highly linear, the nonlinearity of the phase-modulation receiver produces third-order distortion, which limits the SFDR of the link.
Linearization of analog optical links involves modifying the transfer function (input voltage to output voltage) of the link to reduce one or more distortion products. In most analog optical links, the primary distortions are introduced by the transfer function of an interferometer. Specifically, in IMDD links employing Mach-Zehnder intensity modulators this occurs at the transmit end in the modulator whereas in ΦMID links it occurs at the receive end in the interferometric detector. The simplest linearization method involves biasing the interferometer at quadrature where the Taylor expansion of the transfer function is an odd function and thus all even-order distortions are eliminated. However, in links with less than an octave of bandwidth, the odd-order distortions are the most detrimental as their intermodulation products fall in band and thus cannot be filtered out. Thus in sub-octave links, third-order distortion poses the primary limit to the link's SFDR. Therefore reducing or eliminating third-order distortion is highly desirable. The simultaneous elimination of both the second-order and third-order distortions is known as broadband linearization and typically comes at the expense of higher noise.
There are three commonly employed methods of distortion elimination in analog photonic links: analog electronic (feedback/feedforward), digital signal processing, and electro-optic. One example of electronic distortion elimination is predistortion, in which an RF signal with equal and opposite nonlinearity to the transfer function is fed into the modulator. Another example is feed-forward, in which part of the optical output is detected and compared with the input RF signal to generate an error signal. This error signal is inverted and sent to a second electro-optic modulator whose output is added to the first output to produce a more linear output. The second type of distortion elimination involves detecting the output, sending it through an analog-to-digital converter, and using digital signal processing (DSP) to electronically correct the signal. The third type involves connecting multiple modulators or multiple interferometric detectors in either series or parallel to produce an output that is more linear than either of the devices individually. For example, this usually means driving one modulator with a high optical power and low RF power (low distortion) and the other with low optical power and high RF power (high distortion), then combining the signals such that the distortion products cancel while the signal does not, however, due to device constraints this linearization generally comes at the cost of significantly reduced link gain.
Hence, given the above, there is a need for systems and methods capable of reducing or eliminating distortion in phase-modulated analog photonic links while retaining sufficient link gain.